Active Regional Magmatism and Hydrocarbon Reservoirs: Linking Foreland Basin Processes and Hydrothermal Dolomitization

Denis Lavoie
Geological Survey of Canada, Quebec City, QC

World-class hydrocarbon reservoirs are hosted by hydrothermal dolomites (HTD). HTD occur in successions affected by tectono-magmatic foreland basin processes. In eastern North America, Taconian (Upper Ordovician) and Salinian-Acadian (Upper Silurian-Lower Devonian) foreland basins host hydrothermal dolomites.
The Ordovician HTD formed in shallow burial from episodic circulation of high-pressure, warm (135-150°C) and saline (16 to 20 wt% NaCl_equi) fluids along transtensional faults rooted in crystalline basement (⁸⁷Sr/⁸⁶Sr well above Ordovician seawater). Taconian arc volcanism was critical for high geothermal gradients and enhanced deep-seated fluid convection.
The Silurian to Devonian HTD are also of shallow burial origin. Peritidal (Lower Silurian) to platform margin reef (Upper Silurian) to outer shelf (Lower Devonian) carbonate belts are dissected by faults that were reactivated or formed at the Early Silurian onset of the Salinian-Acadian foreland basin. These faults were extensional in Early Silurian but evolved into transpressional at the climax of the Acadian Orogeny in Middle Devonian. These faults were rooted in the older Paleozoic Taconian basement that includes significant mafic units that sourced large amounts of Mg⁺² needed for regional dolomitization. The faults were efficient conduits for episodic high pressure migration of high temperature (180-225°C) and very saline (18 to 26 wt% NaCl_equi) fluids. Fluid convection was enhanced by high geothermal gradients that resulted from back-arc to intraplate volcanism regionally recorded from the Early Silurian to the Early Devonian.
Lower Paleozoic foreland basin tectono-magmatic events were critical for shallow burial hydrothermal alteration of carbonates. The Ordovician and Devonian reservoirs host economic accumulations of hydrocarbons.

AAPG Search and Discovery Article #90072 © 2007 AAPG and AAPG European Region Conference, Athens, Greece